In practicing photolithography to pattern semi-conductor wafers, a common technique is to use a photolithographic tool which has a stage mounted for movement in the x-y plane. A "home" position is established for the tool from which measurements are made to the various target locations on the wafer where exposure is to take place. Typically the target locations will have the targets marked on the wafer designating the desired exposure locations. The stage moves the wafer to bring the target into registration with the exposure tool. After the target is acquired in the exposure field of the tool, normally by means of a laser scanner/detector, final precise alignment of the wafer is accomplished to give exact desired alignment of the exposure field of the tool with the target. The exact location of the photoimage is then recorded by the system based on the measurements made from the "home" position to the final measured position of the target to thereby definitively record and preserve the location of the target. Thus, when any future operations are performed with respect to or otherwise involving the location of the target on the wafer, the actual location of the target will be known.
As the photoimage ground rules, or dimensional tolerances, become tighter and tighter, even small errors in measurements of the locations of the targets can cause significant problems in defining the exact location of the actual exposure. With looser ground rules these would be tolerable, but under the tighter ground rules these errors are becoming unacceptable. One technique for identifying and determining such errors is described in IBM Technical Disclosure Bulletin Vol. 35, No. 3, August 1992, p. 234. This article describes a technique as follows:
Systematic drift of measurement tool repeatability as a function of time is an example of 0th-order drift that is often compensated by a simple offset adjustment. Such techniques based only on periodic re-measurement of known targets have proved inadequate for present technologies. First-order compensation adds curve fitting of measurement system parameters as a function of time to periodic site remeasurement in order to correct measured data. The concept is extendable to higher order error compensation.
The proposed first-order compensation techniques includes the following procedures.
1. Periodically measure a number of sites. The theoretical minimum number is required is three, but five sites are used in practice. PA1 2. First-order compensation cannot be performed in real time, but can only be performed subsequent to collecting all data. PA1 3. Perform compensation on six system parameters. PA1 4. Perform analysis to identify the correct functional form for the six compensation functions.
The set of all five-point drift compensation data is analyzed according to chosen basis functions such as polynomials in x and y. Each set of five data points yields a corresponding set of values for the six first-order parameters. The new set of values are curve fitted according to chosen orthogonal basis functions. The resulting six curves identify the form of the drift as a function of measurement number, or, equivalently, as a function of measurement time.
At each measurement point, the six drift parameters are used to correct the measured data. For example, the x-scale parameter (called Mx) is subtracted from its average value throughout the measurement run, yielding an Mx correction factor. The correction factor is multiplied by the x-location value currently in effect to produce a correction value (in nm) for Mx drift. The remaining parameters are treated similarly. All six correction values are subtracted from the measurement data at that measurement point to obtain compensation for drift in all six first-order parameters. These operations are repeated to compensate for drift in all first-order parameters at all measurement points. The technique described can be extended to include higher order effects if and when such extension is warranted.
This technique while valuable, has certain inherent limitations. The known sites or measuring points referred to in the article are printed or otherwise inscribed on the stage. Thus, when the periodic measurements are made, the measurement to each site will include both system induced errors, and temperature induced errors. These cannot be separately identified or distinguished by this technique. Under tighter ground rules, it becomes necessary to separate these two types of errors since temperature induced errors result in actual movement of the target locations while system or measuring errors given an erroneous reading of where the target is actually located.
For example, temperature variations as little as 0.1 degree C. can result in errors of as much as 50 nm on 200 mm silicon wafers at locations remote from each other; and errors of this magnitude can result in significant misidentification of the location of the exposed areas of the wafer. The so-called system errors are due to conditions changing either within the measuring system or that affect the measuring system. One potential source of such errors may be in the laser interferometer which is typically used for measurements. While this instrument is generally very accurate, it is sensitive to certain environmental changes such as changes in the humidity, etc, causing measurement errors. Other types of measuring errors, the source of which is often unknown, also may occur. These errors are of a different type from those caused by temperature variations. With errors caused by temperature variations, the wafer physically moves due to thermal expansion thus physically moving the target locations. However, with system induced errors, the physical position of the target doesn't move, but rather the measurement of the location by the system changes thus causing a wrong reading of the actual location. In other words, the system may indicate that the target has moved when in fact it has not, or may indicate more or less movement than what has actually occurred. Thus, it becomes desirable to identify, and measure both types of errors so that proper corrections can be made in recording the precise position of the exposure of the wafer.